1. Field of the Invention
The present invention generally relates to an optical scanning apparatus that scans a target scanning surface of a medium to be scanned by a laser beam irradiated from a laser light source, and an image formation apparatus, such as a laser printer, a digital copier, and facsimile equipment equipped with the optical scanning apparatus, using an electronic photographing method.
2. Description of the Related Art
An optical scanning apparatus, which is used by the image formation apparatuses, periodically irradiates a laser beam from a laser light source, which is deflected by a rotating polygon mirror, and an electrostatic latent image is produced on a target scanning surface by the laser beam repetitively scanning in a first scanning direction, to be called the main scanning direction, while the target scanning surface (beforehand charged uniformly by an electrification apparatus) of a photo conductor (scanned medium) moves (rotates) in a second direction, to be called the sub-scanning direction.
The laser beam deflected by the polygon mirror is detected by sync detection means serving as a sync detection sensor at a position that is outside of an image domain (e.g., immediately before a writing start position, or immediately after a writing end position) in the main scanning direction of the target scanning surface.
If the sync detection sensor detects the laser beam, it generates a sync detection signal that specifies the writing start position in the main scanning direction for the laser beam, and outputs the sync detection signal to a laser driving unit, such that the writing position is aligned on the target scanning surface.
After receiving the sync detection signal, and a predetermined time period elapses (i.e., after the laser beam is detected by the sync detection sensor), the laser driving unit starts modulation (ON/OFF) of the laser light source (a laser diode) according to an image signal, and a corresponding laser beam is irradiated.
In this manner, even if there is a division angle error in each reflective face of the polygon mirror, the writing start position can always be aligned at the same position on the target scanning surface, and the writing end position can also be arranged at the same position on the target scanning surface.
Here, a controller, which is not illustrated, transmits image data of a page line by line (scan by scan) as the image signal (video signal) to the laser driving unit. Further, the laser driving unit outputs the image signal to the laser light source in sync with a pixel clock (writing clock), and modulation is performed. The image clock is input via a phase sync unit from a pixel clock generation unit, which constitutes pixel clock generation means and phase setting means.
Here, relations between the pixel clock and its phase change (phase setup) are briefly explained with reference to FIG. 19.
FIG. 19 is a timing chart that shows an example of the relations between the pixel clock and its phase change.
The pixel clock generation unit generates and outputs the pixel clock clkw in the following manner. An oscillator that is not illustrated generates a standard clock (basic clock) clko, repetition frequency of which is n times as high as the pixel clock clkw (4 times in FIG. 19); and a high level (H) and a low level (L) of a signal are toggled every 4 pulses of the standard clock clko by a counting control in sync with the detection signal provided by the sync detection sensor.
The optical scanning apparatus mentioned above is arranged so that writing density of the beam spot becomes uniform when the beam spot of the laser beam is formed on the target scanning surface, and an electrostatic latent image is produced. However, when properties of an optical system change with environmental change etc., errors can occur in writing scale (optical scanning length) per main scanning period of the laser beam generated by the polygon mirror, thereby degrading quality of the image to be output. In order to compensate for the error in the writing scale of the laser beam, a phase change is performed, which shifts the phase of the pixel clock clkw.
Further, there are conventional optical scanning apparatuses that are installed with two or more laser light sources. In the case of such optical scanning apparatuses, the phase change is also performed such that the phase of the pixel clock clkw is shifted in order to compensate for the errors in the writing scale, since the errors (writing scale differences) per scanning period due to the differences in the wavelength of each laser light source can adversely affect the image to be output.
In the optical scanning apparatuses mentioned above, the pixel clock generation unit controls the phase shift of the pixel clock clkw using an external pulse sequence xpls. Specifically, for example, when the pixel clock clkw is generated from the standard clock clko, with the external pulse sequence xpls being input, the pixel clock clkw that is usually generated after 8 pulses of clko, e.g., can be generated after 9 pulses of clko or 7 pulses of clko by changing the number of pulses to be counted. In this manner, the frequency of the pixel clock clkw can be increased to 8/7 times the original frequency (progress control) or decreased to 8/9 times the original frequency (delay control), and the phase of the pixel clock clkw is afterwards shifted. The effect of this is to make the writing scale to be m−7/8 and m+9/8, that is shortened and extended, respectively, where m represents the original time of the whole main scanning line. FIG. 19 demonstrates the case of 1/4 phase shift.
Then, in the conventional optical scanning apparatuses, the above-mentioned external pulse sequence xpls (called simply “pulse” hereinafter) is generated corresponding to a position where the phase change of the pixel clock clkw sequence is desired to occur. For this purpose, the pixel clock generation unit includes a pulse generation unit 99 such as shown by FIG. 20. When a scan in the main scanning direction of the laser beam by the polygon mirror is performed based on a pulse generation interval (period) prd set up in a comparator 101, and a number num that is the number of pulses set up in a comparator 102 by an engine CPU that is not illustrated, the pulse generation unit 99 performs the following operations.
A counter 103 starts a counting operation for counting the number of the pixel clocks clkw when a clear signal xlclr generated from the sync detection signal by a circuit that is not illustrated is received, and when a stop signal is received from the comparator 102, the count operation is stopped.
The comparator 101 compares a count value i of the counter 103 with a predetermined pulse generation interval value prd, and generates the pulse xpls whenever the count value i reaches the predetermined pulse generation interval value prd.
A counter 104 counts the number of the pulses xpls generated by the comparator 101.
The comparator 102 compares a count value j of the counter 104 with the predetermined number of pulses num, and generates a stop signal, when the count value j reaches the predetermined number num.
The operations of the pulse generation unit 99 are explained in detail. As shown in FIG.21, the count values i and j of the counters 103 and 104, respectively, of the pulse generation unit 99 are first reset when the power supply is turned on.
Then, after the clear signal xlclr is received, whenever the counter 103 receives a pixel clock clkw, the counter 103 increments its count, and when the count value i reaches the predetermined value prd, the comparator 101 generates the pulse xpls. The counter 103 resets the count value i to “1” when receiving the pulse xpls.
Further, the counter 104 increments its count when the pulse xpls is received.
Henceforth, the counters 103 and 104, and the comparator 101 repeat the above-mentioned operations. When the count value j of the counter 104 reaches the predetermined value num, the comparator 102 generates a stop signal. In this manner, the operations (called pulse generating operations) of the pulse generation unit 99 are completed.
The pulse sequence generated by the pulse generating operation by the pulse generation unit 99 is as shown by FIG. 22.
In addition to the method mentioned above, another method for generating the fixed pulse sequence is available, wherein the fixed pulse sequence is generated from data output by counting up an address by the pixel clock clkw, using a RAM table, and the like.
There is technology that provides highly precise phase control of the pixel clock for a conventional image formation apparatus (for example, patent reference 1). Further, there is technology that suppresses color shifts in the middle position of an image for an image formation apparatus equipped with two or more photo conductors (for example, patent reference 2).
Patent reference 1: Japanese Application for patent No. 2001-290469 (pp 1-4)
Patent reference 2: JP, 2-291573,A (pp 1-2)
However, according to the pulse generating methods mentioned above using the pulse generation unit 99, the phase change occurs at the same part (the same distance from either edge of the image). In this case, even if there is a phase difference that may not be noticeable in several scanning lines, the output image bears a conspicuous linear pattern produced by image density differences, which appears like a vertical line.
Further, the external pulse sequence xpls is generated irrelevant to the scanning position of the laser beam. If the external pulse sequence xpls is generated when the scanning position of the laser beam is outside of an image domain, the output image is shifted. If the external pulse sequence xpls is generated after the scanning end of the image domain, the external pulse sequence xpls cannot substantially contribute to compensation of the image.
Furthermore, conventionally, the external pulse sequence xpls is generated with only one set of parameters, such as pulse width and cycle. Therefore, even though the phase change is carried out using the external pulse sequence xpls, only one kind of phase control, whether expanding or contracting, can be provided for the whole pixel clock within one scanning period. Therefore, there is a problem that the conventional method cannot provide a solution for a fault that requires a partial compensation of the writing scale.